Financing - Narrative (Details) - USD ($)			1 Months Ended		12 Months Ended
	Jul. 28, 2021	Apr. 16, 2020	Jan. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Sep. 30, 2021	Jul. 27, 2021
Short-term Debt [Line Items]
Short-term debt					$ 375,700,000	$ 0
Commercial Paper
Short-term Debt [Line Items]
Proceeds from Short-term Debt			$ 100,000,000	$ 150,000,000
Commercial Paper
Short-term Debt [Line Items]
Short-term debt					27,200,000	0
Commercial Paper | Commercial Paper
Short-term Debt [Line Items]
Debt instrument, maximum borrowing capacity	$ 650,000,000						$ 550,000,000
Debt instrument, term (in days)	397 days
Notes Payable to Banks
Short-term Debt [Line Items]
Debt instrument, term (in days)		364 days
Line of Credit | Revolving Credit Facility
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 650,000,000						$ 550,000,000
Line of credit facility, term (in years)	5 years
Proceeds from issuance of debt					67,000,000
Long-term debt					0	0
Line of Credit | Revolving Credit Facility | Minimum
Short-term Debt [Line Items]
Debt to capitalization ratio not to exceed	0.65
Line of Credit | Revolving Credit Facility | Minimum | LIBOR
Short-term Debt [Line Items]
Margin rate	0.00%
Line of Credit | Revolving Credit Facility | Minimum | Index Debt Rating
Short-term Debt [Line Items]
Margin rate	0.805%
Line of Credit | Revolving Credit Facility | Maximum
Short-term Debt [Line Items]
Debt to capitalization ratio not to exceed	1.00
Line of Credit | Revolving Credit Facility | Maximum | LIBOR
Short-term Debt [Line Items]
Margin rate	0.50%
Line of Credit | Revolving Credit Facility | Maximum | Index Debt Rating
Short-term Debt [Line Items]
Margin rate	1.50%
364-Day Credit Agreement | Line of Credit
Short-term Debt [Line Items]
Short-term debt					$ 348,500,000	$ 0